Why did I build a power bank?

Why would anyone even try to build a power bank – i.e. an external battery for charging mobile devices – these days? These things are commodity, it’s impossible to compete. Right? Well, that is until you find out that the type of power bank for your application, namely charging a higher-end tablet with 12V input, does not exist cheaply. Looking around for 12V power banks yields a lot of li-ion car jumpstarters (*) and very few actual power banks. Those that exist are pretty expensive and often don’t even perform that well. Let’s run down the list:

Name (linked) Capacity Price Comments
(no-name) Portable PowerBank with 12V & 5V USB Approx. 90Wh $71.99 Big and heavy: weighs 650g. Output power 60W max. Seems to actually be for solar applications. Very little information.
XTPower MP-10000 with dual USB and DC 9V/12V 2A 37Wh $59.90 Output is limited to 2A. Auto turn-off is nice. Weight is ok (300g). Includes lots of connectors.
RAVPower Xtreme 23000mAh Approx. 85Wh $99.99 (normal: $299.99) Pretty big and heavy (600g), very nice design. Super expensive. High output power.
Qualcomm BlitzWolf QC2.0 Approx. 35Wh $26.99 Low output power (12V/1.35A), uses QC2.0 instead of general purpose output

(*) Car jumpstarters will not work, because they have a 3S pack of li-ion cells directly connected to the output, meaning the output actually varies from about 10-12.6V. My tablet (Cube i7 Stylus) and the Microsoft Surface series only accept 12V +/- 5%

Prices exclude shipping. I tried my best to include an example of every ‘category’ of available power bank in this list, but there are obviously hundreds. They fall into four general categories:

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